Potential Biases in the Data Because about 40 percent of the store/home pairs of locations could not be fully geocoded, it is important to examine whether there may be biases implicit in the resulting data. For examination of this issue, Table E.1 displays two sets of data on distance to the store usually shopped: one set based on the geocoding and the other on a direct question asked during the interview. Comparison of the two columns shows that the geocoded data clearly imply shorter distances, on average, than the direct interview data. This suggests the possibility that the stores that could not be geocoded may be disproportionately the ones at greater distances from respondents. Based on the interviewing and coding experience, it is likely that this type of bias may indeed have occurred, since it tended to be harder for respondents to supply detailed address information for stores that were outside their own neighborhoods. This potential bias needs to be taken into account in interpreting data based on the geocoding. It is unlikely, however, that it reverses any conclusions made in the report. This issue is examined further in the next section.
Reassessment of Whether Respondents Frequently Travel Farther than the Nearest Supermarket to Shop, in Light of the Possible Biases in the Geocoded Data The analysis in Chapter IV concludes that FSP participants frequently travel farther than the nearest supermarket for their food shopping. However, as noted in that discussion, the conclusion may be influenced by the fact that direct survey responses about distances to the store most often used are being compared with geocoded information about the nearest supermarket. (Use of the two different types of data maximized available sample sizes. No direct interview data are available on distance to the nearest supermarket.)
E.4